This application includes certain subject matter contained in an application of Herr, Ser. No. 09/934,493, filed Aug. 22, 2001, concurrently herewith entitled, xe2x80x9cDouble Flux Quantum Superconductor Driver, xe2x80x9d and the prior application of Herr, Abelson and, Kerber, Ser. No. 09/882,979 filed Jun. 15, 2001, entitled xe2x80x9cCapacitor for Signal Propagation Across Ground Plane Boundaries in Superconductor Integrated Circuits,xe2x80x9d both of which are copending herewith, and which are assigned to the assignee of the present application.
This invention relates to superconductor devices and, more particularly, reduction of bias current demand required in superconductor integrated circuits to power large numbers of Josephson Junctions contained within the integrated circuits, and to coupling circuits for superconductor single flux quantum pulses.
Metals, metal alloys and ceramics found to exhibit zero electrical resistance are commonly referred to as superconductors. Typically, those superconductors don""t attain the superconductive state unless cooled to extremely low temperatures, referred to as cryogenic temperatures. Each such superconductor material possesses a unique cryogenic temperature, referred to as the transition temperature (xe2x80x9cTcxe2x80x9d), at which the respective metal and metal alloy becomes superconducting, changing in electrical resistance from a measurable or relatively high value of resistance to zero. One known superconductor is niobium, a refractory metal, which transitions to a superconducting state at a temperature of 9.2 Kelvin.
Superconductor digital electronic devices have previously been constructed of superconductor metals and the functionality of such devices demonstrated. As example, with a zero-resistance characteristic during superconductivity, electrical current induced into a loop formed of the superconductor metal, refrigerated below the transition temperature of the metal, persists indefinitely. With appropriate drivers and sensors, the foregoing loop may serve as a digital memory. When the direction of the current induced in the loop is in a clockwise direction the memory state may represent a xe2x80x9c1xe2x80x9d digital bit; when the direction of induced current is counterclockwise, the memory state may represent the bit xe2x80x9c0xe2x80x9d.
Superconductor digital electronics devices have been fabricated as integrated circuits on a silicon wafer using the photo-lithographic mask and etch techniques or other known techniques most familiar to those in the semiconductor industry. Such superconductor integrated circuit devices provide the desired functionality in a very small package or chip. Superconductor devices operate at very high speeds, as example, 100 GHz to 770 GHz, and very low power, which is unattainable with present semiconductor devices. Because of the high speeds of operation and low power requirement, superconductor electronic devices remain attractive for many applications.
A principal element to the construction of a superconductor digital electronic device is the Josephson junction. The Josephson junction is formed, as example, of two layers of superconductors, such as niobium, separated by a very thin layer of electrical insulation, such as aluminum oxide. When cooled to the transition temperature and biased with DC current below a certain xe2x80x9ccritical currentxe2x80x9d, (xe2x80x9cIcxe2x80x9d) the Josephson junction is superconducting and the junction conducts current without developing a voltage drop there across and without dissipation of energy, exhibiting no electrical resistance. Consequently, the junction does not produce heat, which is a significant advantage for integrated circuits. If biased above the critical current, the Josephson junction produces an RF signal, consisting of a series of pulses at RF frequencies. Thus, the critical current is a boundary at which the electrical properties of the junction changes as described.
Superconductor circuits utilize the foregoing property of the Josephson junction to regenerate single flux quantum (xe2x80x9cSFQxe2x80x9d) pulses. The time integral of the voltage of a single flux quantum pulse is a physical constant approximately equal to 2.07 millivolt picoseconds or, in alternate terms, 2.07 milliamp picohenry. When an SFQ pulse is applied to a Josephson junction that is properly DC biased below the critical current, the current produced by the SFQ pulse when added to the DC bias current may cause the Josephson junction to briefly exceed the critical current. The Josephson junction then undergoes a 360 degree shift in quantum phase or, as otherwise termed, electronically xe2x80x9cflips-overxe2x80x9d. In undergoing that shift the Josephson junction generates an SFQ pulse in response to the applied SFQ pulse.
In superconducting integrated circuit (xe2x80x9cICxe2x80x9d) devices containing multiple Josephson junctions, the junctions are formed on a common superconductor metal layer, referred to as a ground plane, deposited over an insulator substrate, such as silicon, a readily available and inexpensive material. The multiple Josephson junction devices may be logically divided into groups of two or more junctions, the groups referred to as xe2x80x9cSQUIDSxe2x80x9d (an acronym for superconducting quantum interference device). For example, a single flux quantum pulse transmission line, referred to as a Josephson transmission line, may be formed of a number of SQUIDS arranged in serial order, each SQUID containing two Josephson junctions connected electrically in parallel in a superconducting loop, the latter also sometimes referred to as a Josephson loop.
A single flux quantum pulse applied to the input of the Josephson transmission line (xe2x80x9cJTLxe2x80x9d), may be said to propagate along the transmission line to the output, moving from SQUID to SQUID in that line, and thence to the electrical load connected to the output of the transmission line. In fact, the SFQ pulse is regenerated at each Josephson junction (stage), which can produce current and power gain. The transmission line may in total contain two or more Josephson junctions, the number of Josephson junctions (and SQUIDS) that form the transmission line can be increased to traverse the desired distance.
At present, powering (e.g. biasing) superconducting single flux quantum circuits requires very low DC voltage, but appreciable current. Typically, the DC bias supply must supply about 0.1 mA to each Josephson junction contained within a superconducting IC. With many such junctions (or SQUIDS) in a superconductor device, the total bias current is cumulative and in total is very large. Existing techniques for powering Josephson Junctions in superconductor circuits (e.g. SFQ circuits) are based on a parallel bias, that is, bias current supplied to the circuits in parallel, in which all the superconductor digital gates and functional blocks thereto have a common circuit ground. Increasing the number of gates increases the current demand required of the power bus and the DC bias power supply that supplies the power to that bus.
For superconductor circuits of several thousand gates (e.g. SQUIDS) or larger, more than one ampere of total current is required, which is relatively large for integrated circuit devices. Requiring large current at low voltage, even at power levels as low as one milliwatt, presents at least two disadvantages. First, semiconductor power converters presently available do not deliver an ampere of current at one millivolt in voltage as efficiently as they deliver one milliamp current at one volt of voltage. Secondly, the transmission of larger currents from an external current supply, positioned in the ambient temperature, to the cryogenic package containing the superconductor circuits implies electrical conductors for the power bus that are large in cross-section.
The large cross section of the power bus conductors, in addition to conducting current, provides a thermal path from the ambient into the cryogenic package that is of greater thermal conductivity than with bus""s of small cross-section. Due to the greater thermal conductivity, more heat could be conducted into the cryogenic package. The addition of heat to the cryogenic package is obviously undesirable, since the heat increases the requirements for refrigeration to maintain the circuits in the superconductive state, also lowering efficiency. As an advantage the present invention is able to power large numbers of SQUIDS without increasing the cross-sectional area of the power bus.
The problem of powering large numbers of SQUIDS in a superconductor integrated circuit while maintaining the lowest level of current demand on the power supply (or power supplies) was earlier recognized by one of the co-inventors, who, with others, jointly conceived a new biasing arrangement for the SQUIDS, one that separated the paths for transmission of SFQ pulses (the AC path) from the paths for supplying bias current. In that new biasing arrangement multiple SQUIDS of a large Josephson transmission line are supplied with DC bias current in a series a circuit, instead of the existing practice of being supplied that bias current in parallel circuit. The foregoing invention is the subject of a copending application for patent, Ser. No. 09/882,979, filed Jun. 15, 2001, by Herr, Abelson and Kerber (the xe2x80x9c""979 Herr et al applicationxe2x80x9d), entitled, xe2x80x9cCapacitor for Signal Propagation Across Ground Plane Boundaries in Superconductor Integrated Circuitsxe2x80x9d, assigned to the assignee of the present application.
That new biasing arrangement reduces the total electrical DC bias current requirement (e.g. current demand) that was previously necessary to properly bias all the SQUIDS contained in a Josephson transmission line. Prior to the foregoing invention, the SQUIDS of a Josephson transmission line were supplied with DC bias current in parallel, producing a current demand that was a multiple of the bias current required by a single SQUID.
Although engineers in the superconductor art are familiar with series electrical circuits, the prior art did not contain a practical way to power the multiple SQUIDS in series and still obtain a functional superconductor device. The conundrum was that one could DC isolate the SQUIDS from one another and supply the bias current to the SQUIDS in series circuit, without being able to transfer an SFQ pulse from one SQUID to another, rendering the formed Josephson transmission line dysfunctional; and one could transfer a SFQ pulse from SQUID to SQUID in a functional Josephson transmission line, but only if the SQUIDS were not DC isolated from one another, and, hence, were supplied with DC bias current from the power supply in parallel.
The ""979 Herr et al application disclosed a means for reducing or eliminating the self-inductance in the wiring to a capacitor installed in a superconductor integrated circuit device, specifically a Josephson transmission line. The structure in effect produces a negative inductance to counter-act the inherent self-inductance of the capacitor wiring, producing, ideally, a net zero self-inductance. Applying that discovery, an SFQ pulse could then successfully pass through a capacitor in a superconductor circuit, avoiding the dominant absorptive effect of the series self-inductance of the capacitor wiring inherent at the propagation speeds of the SFQ pulses.
Incorporating the new capacitors of the ""979 Herr et al application in series in the SFQ pulse transmission path (e.g. the AC path) of the Josephson transmission line, the AC transmission path becomes distinct from the DC bias current path. Separate SQUIDS in the Josephson transmission line could thereby be maintained electrically DC isolated from one another. Thus the DC isolated SQUIDS may then be wired in DC series circuit; and the DC bias current supply need supply bias current to that series circuit of multiple SQUIDS at the level of current required to bias a pair of Josephson Junctions in one of the SQUIDS, instead of the higher levels previously required. Power supplies that deliver smaller levels of current at higher voltage are more practical, readily available, less expensive and smaller in size that those that are required to supply very high current at low voltage. The foregoing innovation in biasing rendered Josephson transmission lines and any other superconductor device containing large numbers of Josephson Junctions more practical.
Like capacitor coupling, those in the field also recognize the transformer as a means to couple an AC signal from one circuit location to another and as a way to concurrently DC isolate the one circuit from the other. A transformer contains a primary winding and a secondary winding which are DC isolated from one another. If the side of the circuit that functions with the secondary is connected to a ground that is isolated from the ground of the first circuit location, an AC signal may effectively pass through the transformer, but not the DC, which is blocked. Even possessed of such basic knowledge, no one has heretofore been known to accomplish the foregoing in a superconductor circuit and reduce the level of bias current to that circuit. As an advantage the present invention accomplishes both functions.
Accordingly, a principal object of the present invention is to reduce the level of electrical current required to power a superconductor device.
A further object of the invention is to significantly reduce the DC current draw required to power superconductor ICs containing large numbers of Josephson junctions.
Another object of the invention is to permit single flux quantum pulses to propagate across ground plane boundaries in superconductor integrated circuits.
Still another object of the invention is to provide a new biasing arrangement for superconductor integrated circuits; and
An ancillary object of the invention is to employ a superconductor transformer to provide DC isolation between portions of a superconductor circuit.
In accordance with the foregoing objects and advantages, the level of bias current required by a superconductor integrated circuit is lowered by separating the circuit into portions that have separate ground planes and supplying the bias current to the circuit portion in one ground plane in series with that for the circuit portion in the other ground plane. To maintain DC isolation between those circuit portions, a pair of inductively coupled SQUIDS that define a DC transformer is provided so that SFQ pulses inputted to the circuit move across the separate ground planes through the SQUIDS. A combiner reconstitutes and outputs the SFQ pulses at the circuit output. To provide inductive coupling the DC transformer includes a primary and an isolated secondary winding.
In accordance with a more specific aspect to the invention, a superconductor flip-flop is used to split a train of SFQ pulses into separate pulse trains of odd numbered SFQ pulses and even numbered SFQ pulses, respectively. The latter pulse trains are applied to respective inputs to the primary of a DC transformer, which effectively lies in the first ground plane, and those pulse trains are coupled to a DC isolated secondary, which effectively lies in the second ground plane. The two pulse streams thereby bridge the ground planes. A combiner recombines the two pulse streams into a single pulse stream, thereby reconstituting the original pulse stream that was inputted to the toggle flip-flop. The components that lie within the first and second ground planes, respectively, include appropriate bias inputs. The first circuit ground is coupled to the bias inputs of those components that lie in the second ground plane, and the external source of bias connects between the bias inputs of those components that lie in the first ground plane and the second ground plane, thereby supplying bias current to the two sets of components in series.
The foregoing and additional objects and advantages of the invention, together with the structure characteristic thereof, which were only briefly summarized in the foregoing passages, will become more apparent to those skilled in the art upon reading the detailed description of a preferred embodiment of the invention, which follows in this specification, taken together with the illustrations thereof presented in the accompanying drawings.